Space based and ground based optical systems needed for the controlled transmission of radiated power require compensation for atmospheric effects on propagating laser beams and on imaging systems. The atmosphere distorts the beam's wavefronts, yielding a beam which is difficult to focus. As the beam propagates through the atmosphere, the phase uniformity which initially characterized the beam is lost because of interactions with the atmosphere or other optical elements in the optical train. To maximize power delivery to a target, it is necessary to have wavefront phase uniformity across a plane transverse to the propagation direction of the beam. Beams which have nonuniform (aberrated) transverse phase profiles cannot be brought to as sharp a focus.
Efforts to compensate for this distortion have lead to the development of adaptive optical elements that have deformable optical surfaces configured to approximate the conjugate shape of an incoming beam's wavefront distortions. The outgoing conjugate beam is similarly distorted by the atmosphere to have near perfect wavefronts. An example of an adaptive optical element is disclosed in the commonly owned, co-pending U.S. patent application Ser No. 114,540 entitled "Extendable Large Aperture Phased Array Mirror System".
A near perfect wavefront is characterized by phase uniformity across a plane transverse to the axis of beam propagation. Apparatus commonly used for wavefront phase analysis include a conventional lateral shearing interferometer to measure wavefront slope data, and superheterodyne mechanisms which compare the wavefront phase of a local oscillator, such as a laser, to that of a sampled wavefront. A lateral shearing interferometric phase measurement technique requires complicated and time consuming solutions of simultaneous equations, or an analog network of resistors to obtain wavefront phase data from wavefront slope data. The alternative superheterodyne technique mandates phase locking of two lasers which is very difficult to accomplish in non-laboratory environments.
Other known devices used for wavefront phase analysis include the phase measuring interferometer disclosed in U.S. Pat. No. 4,575,247, entitled "PHASE MEASURING INTERFEROMETER." The apparatus disclosed therein comprises a modified point diffraction interferometer which separates an incoming laser beam into two orthogonally polarized components. The beam components are propagated through an optical frequency shifter having a rotating half-wave plate. A pair of beams exits the frequency shifter which differ in frequency by four times the angular frequency of the half-wave plate. Consequently, the polarization vectors of both frequency shifted beams are colinear with the beam propagative axis. The '247 interferometer combines the two beams to produce an interference pattern consisting of alternating light and dark bands travelling in one direction. A conventional phase-detector measures the phase difference between a reference position and a plurality of other positions in the wavefront pattern. Each of the above wavefront phase measuring techniques is undesirably complex to implement.
It would be advantageous to have an adaptive optical system with local wavefront phase sensing which does not depend on polarization of either the input or reference waves, and which does not measure phase shift in an interference pattern with respect to a reference position. The present invention is drawn towards such system.